Fuel cells are electricity generation systems that directly convert the chemical energy of oxygen and the hydrogen in hydrocarbons such as methanol, ethanol, and natural gas to electrical energy.
Fuel cell systems consist of a fuel cell stack, a fuel processor (FP), a fuel tank, and a fuel pump. The fuel cell stack is the main body of a fuel cell, and includes several to several tens of unit cells, each including a membrane electrode assembly (MEA) and a separator (or bipolar plate).
The fuel pump supplies fuel in the fuel tank to the fuel processor. The fuel processor produces hydrogen by reforming and purifying the fuel and supplies the hydrogen to the fuel cell stack. The fuel cell stack receives the hydrogen and generates electrical energy by electrochemical reaction of the hydrogen with oxygen.
A reformer of the fuel processor reforms hydrocarbon fuel using a reforming catalyst. Since a hydrocarbon fuel typically contains one or more sulfur compounds, and since the reforming catalyst is easily poisoned by sulfur compounds, it is necessary to subject the hydrocarbon fuel to desulfurization prior to the reforming process in order to remove sulfur compounds prior to reforming the hydrocarbon fuel.
FIG. 1 is a schematic flow diagram illustrating fuel processing in a fuel processor used in a conventional fuel cell system.
Hydrocarbon reforming produces carbon dioxide (CO2) and a small amount of carbon monoxide (CO) as by-products, together with hydrogen. Since CO acts as a catalyst poison in electrodes of the fuel cell stack, reformed fuel should not be supplied to the fuel cell stack until a CO removal process has been carried out. It is desirable to reduce the CO levels to less than 10 ppm.
CO can be removed using a high-temperature shift reaction represented by Reaction Scheme 1 below.

A high-temperature shift reaction is performed at a temperature of 400 to 500° C. Generally, a high-temperature shift reaction is followed by a low-temperature shift reaction at a temperature of 200 to 300° C. Even after these reactions are performed, it is very difficult to reduce the CO levels to less than 5,000 ppm.
To solve this problem, a preferential oxidation reaction (referred to as the “PROX” reaction) represented by Reaction Scheme 2 below can be used.

However, a side reaction represented by Reaction Scheme 3 occurs together with the PROX reaction.

Thus, in order to maintain a high level of H2 while reducing CO, it is important to increase the rate of the PROX reaction represented by Reaction Scheme 2 and enhance the reaction selectivity for the PROX reaction by minimizing the side reaction represented by Reaction Scheme 3 as well.
Another serious potential problem is that a methanation reaction may occur between CO to be removed and reformed hydrogen, as represented by Reaction Scheme 4 below. It is important to inhibit this reaction since even limited methanation reactions can lead to a significant decrease in the hydrogen concentration and can affect the efficiency of the entire reforming process.

Conventional catalysts for oxidizing carbon monoxide in the PROX reaction have low reaction selectivity. Furthermore, when conventional catalysts are used, the methanation reaction partially occurs and the conventional catalysts lose reactivity by becoming reoxidized by oxygen in the reaction device during the catalytic operations or during intervals between operations.
Therefore, it is necessary to develop a PROX catalyst that has a high reaction activity, excellent reaction selectivity and a good reduction-oxidation property by re-reduction of oxidized catalyst and that does not support a methanation reaction.